Question

How do I set the Windows system clock to the correct local time using C#?

Answer 1

You will need to P/Invoke the SetLocalTime function from the Windows API. Declare it like this in C#:

[DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
internal static extern bool SetLocalTime(ref SYSTEMTIME lpSystemTime);

[StructLayout(LayoutKind.Sequential)]
internal struct SYSTEMTIME
{
    public ushort wYear;
    public ushort wMonth;
    public ushort wDayOfWeek;    // ignored for the SetLocalTime function
    public ushort wDay;
    public ushort wHour;
    public ushort wMinute;
    public ushort wSecond;
    public ushort wMilliseconds;
}

To set the time, you simply initialize an instance of the SYSTEMTIME structure with the appropriate values, and call the function. Sample code:

SYSTEMTIME time = new SYSTEMTIME();
time.wDay = 1;
time.wMonth = 5;
time.wYear = 2011;
time.wHour = 12;
time.wMinute = 15;

if (!SetLocalTime(ref time))
{
    // The native function call failed, so throw an exception
    throw new Win32Exception(Marshal.GetLastWin32Error());
}

However, note that the calling process must have the appropriate privileges in order to call this function. In Windows Vista and later, this means you will have to request process elevation.

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Alternatively, you could use the SetSystemTime function, which allows you to set the time in UTC (Coordinated Universal Time). The same SYSTEMTIME structure is used, and the two functions are called in an identical fashion.